dc.description.abstract | Cold-formed hollow sections are attractive for many different construction parts due to
their high strength and relatively low cost. Due to the production method, such profiles
often contain large residual stresses. It is important to know how this affects the strength
of the material. This thesis presents an experimental study of various material properties
and material parameters in a cold-formed hollow profile of the type 80x80x6 with the
steel type S420MH. The effect of the residual stresses on the plastic bending capacity is
investigated. In addition, the value and quality of the material parameters are examined
through numerical modeling of three- and four-point bending tests.
Heat treatment in the form of stress relief annealing was performed on a set of profiles
to release residual stresses. This made it possible to compare properties and parameters
between two profiles, with and without residual stresses.
Quasi-static UT-tests performed on specimens from the corner and the middle edge of the
profile were performed. A camera made it possible to continuously measure the diameter
of the necking in the specimens. The results from these tests showed that the residual
stresses increase the yield stress but reduce the ductility. Several larger specimens were
also taken out. These were painted with a spreckled pattern and continuously monitored
using DIC. Based on this, it became possible to extract true stress and true strain. This
formed the basis of the material curve for the profile.
To investigate the effect of the residual stresses in an entire beam, quasi-static bending
tests were carried out for profiles with and without heat treatment. The two types of
profiles were subjected to two different types of tests: three- and four-point bending tests,
where one and two plastic hinges, respectively, are to be expected. The results of the
bending tests were, as in the UT tests, that the material with residual stresses has a higher
yield stress but lower ductility. This was particularly clear in the four-point bending test
where two plastic hinges occur, here the profile which had been heat treated showed a
significantly better ability to redistribute forces from one plastic hinge to the other. The
material curve was taken into ABAQUS and used to model an element model of the
quasi-static bending tests. The force vs displacement curve was compared for models and
the physical test and the match between the two was considered to be relatively good. | |